1. Field of the Invention
This invention relates to an image heating apparatus suitable for use as a fixing apparatus carried on a copying machine or a printer using a recording technique such as an electrophotographic recording process or an electrostatic recording process.
2. Description of the Related Art
In image forming apparatuses such as electrophotographic copying machines and printers, a heat fixing apparatus of a heat roller type has heretofore been widely used as a fixing apparatus for heating and fixing an unfixed toner image indirectly (transfer) or directly formed and borne on a recording material (paper) as a material to be heated by suitable image forming process means such as electrophotographic process means as a permanent fixed image on the surface of the recording material.
In recent years, from the viewpoints of quick start and energy saving, a heat fixing apparatus of a film heating type has been put into practical use. The fixing apparatus of the film heating type is proposed, for example, in Japanese Patent Application Laid-open No. S63-313182, Japanese Patent Application Laid-open No. H2-157878, Japanese Patent Application Laid-open No. H4-44075 and Japanese Patent Application Laid-open No. H4-204980.
That is, a flexible sleeve (hereinafter referred to as the fixing film) is nipped between e.g. a ceramic heater as a heating member and a pressure roller as a pressure member to thereby form a pressure contact nip (hereinafter referred to as the fixing nip portion), and a recording material having an unfixed toner image formed and borne thereon is introduced into between the fixing film and the pressure roller in the fixing nip portion and is nipped and conveyed together with the fixing film, whereby the unfixed toner image is fixed on the surface of the recording material by the pressure force of the fixing nip portion while the heat of the ceramic heater is given thereto through the fixing film.
This fixing apparatus of the film heating type can constitute an apparatus of an on-demand type by the use of members of low heat capacity as the ceramic heater and the film, and only during the execution of image forming, the ceramic heater as a heat source can be electrically energized to thereby generate heat to a predetermined fixing temperature, and there are the advantages that the waiting time from the turn-on of the power supply switch of the image forming apparatus until a state in which image forming can be executed is short, and that the power consumption during standby can be greatly curtailed.
When the film is used in the film heating process, shifting in the generatrix direction of the film may sometimes occur, and it is difficult to severely control this. So, in Japanese Patent Application Laid-open No. H4-44075, Japanese Patent Application Laid-open No. H4-204980, etc., it is proposed to wind film loosely to thereby reduce the shifting force of the film and receive the end portion of the film by the film end portion regulating surface (hereinafter referred to also as the “regulating surface”) of a flange, and effect the regulation of the shifting.
FIGS. 8 and 9A of the accompanying drawings show an example of shifting regulating structure for the end portion of film in a heat fixing apparatus. The shifting regulating structure shown in FIGS. 8 and 9A is provided at locations opposed to the opposite end portions of fixing film. FIG. 9B of the accompanying drawings is an enlarged view of the portion Z of FIG. 9A.
In FIG. 8, the reference numeral 21 designates a guide member for guiding the inner surface of sleeve-shaped fixing film 25. Fixing flanges 60 are fitted to the opposite end portions of the guide member 21. Each flange 60 is provided with an inner peripheral sliding portion 60a opposed to an end portion of the fixing film 25 for guiding the rotational locus of the film, a wall 60b substantially orthogonal to the film sliding surface (outer peripheral surface of the inner peripheral sliding portion so as to regulate the end portion position (end surface) of the fixing film 25, etc. The fixing film 25 is in pressure contact with a pressure roller, not shown, to thereby form a fixing nip portion, and receives a rotational force from the rotatively driven pressure roller and is rotated thereby. In a case where during the rotation, shifting in the generatrix direction occurs to the fixing film 25 as indicated by arrow A in FIG. 9A, the end portions (end surfaces) of the fixing film 25 are stopped by the walls 60b of the fixing flanges 60, and the fixing film 25 continues to rotate in that position.
In the conventional heat fixing apparatus as described above, however, there have arisen such problems as will be described below.
As the heat-resistant fixing film to be used in the above-described film heating process, use is made of a material such as heat-resisting resin film such as polyimide or PEEK, or metal film such as Ni electrocast film or stainless seamless film. The reason why these materials are used is that they are high in durability in terms of both heat resistance and strength even under a condition under which the fixing film slides while being directly in pressure contact with a heater (heat generating member) generating heat to 200° C. or higher.
In the aforedescribed heat fixing apparatus of the film heating type, however, even in a case where the material of high durability as described above is used for the fixing film 25, when besides the unevenness of the degree of cylindricality and thickness of the fixing film, the fluctuation of the diameter of the pressure roller which is a pressure member or when the balance of the pressure force is not sufficiently secured, the fixing film is biased to one end portion side of the heater in the longitudinal direction thereof (the axial direction of the pressure roller, and soon the end portions of the fixing film strike against the walls 60b of the fixing flanges 60. The material of the flanges 60 is resin, and the flanges are molded by a metal mold and therefore, as shown in FIG. 9B, a portion R exists in the intersecting portion between the inner peripheral sliding portion 60a and the wall 60b. Accordingly, if the fixing film continues to rotate while being in contact with the walls 60b, a force continues to be applied to the end portions of the fixing film 25 in a state in which it is pushed open from the inner surface side thereof to the outer surface side thereof, by the portion R in part working which is in the root portion, i.e., the intersecting portion, of the inner peripheral sliding portions 60a of the fixing flanges 60 with the walls 60b and therefore, fracture occurs, and finally the fixing film is broken into a strip shape, and this has sometimes caused such problems as a faulty image, the faulty fixing of a toner and paper jam.